LNCs are sometimes also referred to in the literature as non-selective catalytic reduction (NSCR) catalysts, hydrocarbon selective catalytic reduction (HC-SCR) catalysts, lean NOx reduction catalysts, “DeNOx catalysts” and NOx occluding catalysts.
In lean NOx catalysis, hydrocarbons (HC) react with nitrogen oxides (NOx), rather than oxygen (O2), to form nitrogen (N2), carbon dioxide (CO2) and water (H2O) according to reaction (1):{HC}+NOx→N2+CO2+H2O  (1)
The competitive, non-selective reaction with oxygen is given by reaction (2):{HC}+O2→CO2+H2O  (2)
A number of catalysts are known for selectively promoting the reaction (1) including platinum (Pt) on alumina (Al2O3), copper (Cu)-substituted zeolite such as Cu/ZSM-5 and silver (Ag) supported on Al2O3 (see e.g. EP 0658368).
Pt/Al2O3 is active in a relatively narrow temperature window at relatively low temperature (peak activity ˜250° C.). Another problem is that it is relatively unselective for N2 formation in that it favours the formation of N2O over N2 at relatively low temperatures. N2O is a potent greenhouse gas (at least 200 times more potent than CO2) and accordingly its release into the atmosphere is undesirable.
Zeolite-based LNCs, such as Cu/ZSM5, and Ag/Al2O3 LNCs have wider temperature windows of activity than Pt-based LNCs and also operate at higher temperatures (peak activity of about 400-450° C. and above).
It would be desirable to develop an exhaust system for treating exhaust gas of a lean burn internal combustion engine for selectively reducing NOx to N2 in the presence of a suitable reductant over a relatively broad temperature window of activity without producing N2O.
We have investigated the known silver- and zeolite-based LNC's and we have found that their low temperature activity is improved when the reductant is treated with a partial oxidation catalyst. We believe that this step produces an oxygenated hydrocarbon feedstock. Furthermore, we believe that the activity of Ag/Al2O3 is suppressed by coke formation, which may be one reason why it has not found wider acceptance in the industry.
A partial oxidation catalyst (POC) is a catalyst that promotes the partial oxidation of hydrocarbons in exhaust gas of a lean-burn internal combustion engine to carbon monoxide (CO), hydrogen gas (H2) and partially oxygenated hydrocarbon species, as opposed to complete oxidation to H2O and CO2.
EP 1008379 describes a method and device for treating NOx in an internal combustion engine exhaust system comprising a POC upstream of a NOx trap, wherein hydrocarbons introduced into the exhaust system upstream of the POC are converted to CO and H2 for regenerating the NOx trap.
EP 0761289 describes an exhaust gas cleaner and method for removing NOx by reduction from a lean burn internal combustion engine exhaust gas. According to the disclosure, nitrogen-containing compounds such as alkyl nitrites, ammonia etc. and aldehydes are produced on e.g. Ag/Al2O3 which are subsequently reduced by being brought into contact with a second catalyst of: (a) titania; (b) a titanium component supported on an inorganic oxide; (c) an inorganic oxide carrying at least one first component selected from the group consisting of copper, nickel, silver, cobalt and compounds thereof and at least one second component selected from the group consisting of titanium, zirconium and compounds thereof and a third component selected from the group consisting of alkali metal elements and rare earth elements; and (d) an inorganic oxide carrying a titanium component and at least one platinum group metal. The technique of non-selective catalytic reduction is specifically distinguished in the background to the disclosure.